The use of fluxes, particularly rosin fluxes, is a virtually inescapable aspect of processes requiring soldering, i.e., the making of mechanical, electromechanical or electronic connections with metal. Fluxes may be applied to the metal surface separately or as part of the solder paste. Several functions are achieved by flux application: the flux chemically and/or physically combines with undesirable "debris" on the metal surface, such as oxides and sulfides, and prevents oxide formation during the solder heating process. It also aids in reducing surface tension of the molten solder. After soldering is complete, however, the flux residues must be removed, since they may be corrosive, or interfere with the normal functioning of the apparatus to which they have been applied. The removal of these compounds is frequently not a routine matter, and usually cannot be achieved by a simple water wash. For example, in the fabrication of printed wiring boards and/or printed circuit boards, soldering fluxes are first applied to the substrate board material to ensure firm, uniform bonding of the solder. These soldering fluxes fall into two broad categories: rosin fluxes and non-rosin, or water soluble, fluxes. The rosin fluxes, which are generally less corrosive and have a much longer history of use, are still widely used throughout the electronics industry The water soluble fluxes, which are a more recent development, are being used increasingly in consumer products applications. Because they contain strong acids and/or amine hydrohalides and are thus corrosive, the water soluble soldering fluxes can cause circuit failure if residual traces of the material are not carefully removed. For that reason military specifications require the use of rosin fluxes. Even the use of rosin soldering fluxes, however, can lead to premature circuit failure due to decreased board resistance if traces of residual flux are not removed following soldering.
Fabrication of circuit boards is just one example of the use of fluxes. Other processes in which flux residues are produced, and in which removal is necessitated, include circuit board assembly, component packaging, and board clean-up after in-service maintenance. The joining of interconnect devices, e.g. computer connections with cables, also may require the use of solder and solder flux. More traditional uses of course include pipe-fitting, or any other procedure which requires the joining of metal surfaces. The most common flux used in both electronic and electromechanical joining are rosin fluxes, and in such uses, the removal of the flux is strongly preferred, if not absolutely necessary.
While water soluble fluxes can be easily removed with warm, soapy water, the removal of rosin flux from printed wiring boards has traditionally been carried out with the use of chlorinated hydrocarbon solvents such as 1,1,1,-trichloroethane, trichloromonofluoromethane, methylene chloride, trichlorotrifluoroethane, or mixtures or azeotropes of these solvents. These solvents are undesirable, however, because they are toxic or otherwise environmentally objectionable. Thus, their use is subject to close scrutiny by the Occupational Safety and Health Administration (OSHA) or other regulatory bodies, and stringent containment equipment must be used. Moreover, if released into the environment these solvents are not readily biodegradable and are thus hazardous for long periods of time.
Alkaline cleaning compounds known as the alkanol amines, usually in the form of monoethanolamine, have been used for rosin flux removal as an alternative to the toxic halogenated hydrocarbon solvents. These compounds chemically react with rosin flux to form a rosin soap through the process of saponification. Other organic substances such as surfactants or alcohol derivatives may be added to these alkaline cleaning compounds to facilitate the removal of such rosin soap. Unfortunately, these compounds, like the water soluble soldering fluxes, have a tendency to cause corrosion of the surfaces and interfaces of printed wiring boards if they are not completely and rapidly removed during the fabrication process.
In another approach, Bakos et al. (U.S. Pat. No. 4,276,186) have used mixtures of N-methyl-2-pyrrolidone and a water miscible alkanolamine to remove solder flux and solder flux residue from integrated circuit modules. These mixtures were also said to be useful for removing various cured synthetic organic polymer compositions such as cured polyimide coating compositions from integrated circuit chip modules.
Another alternative to the use of halogenated hydrocarbons has also been described in U.S. Pat. No. 4,640,719 to Hayes et al. Described therein is a method for cleaning circuit boards which employs terpenes as cleaning agents. Due to the above noted environmental conditions, there is an increasing commercial demand for highly effective circuit board cleaners having a variety of desirable characteristics, such as non-toxicity, high flash point and low environmental impact which enhance their usefulness under a variety of circumstances. The compositions of the present invention, comprising dibasic esters, provide the desired characteristics in their low toxicity relative to halogenated hydrocarbons, as well as having a relatively high flash point. Moreover, the present compositions do not have a harmful effect on the earth's stratospheric ozone layer; depletion of the ozone layer is another problem which is encountered with the use of halogenated hydrocarbons. Further, esters of the present invention generally have a relatively low vapor pressure, and are therefore less volatile, providing the advantage of lower volatile organic compound (VOC) pollutant emissions, than prior art compositions. These compositions are also useful in removing other difficult organic residues, such as printing inks and resists. Thus, the present method and compositions provide cleaning ability equivalent to those known in the art, while also providing advantageous characteristics which have not generally been found in most prior art methods and compositions.